1. Field of the Invention
The present invention relates to a power supply device in which a light source lamp, particularly, a vacuum discharge-based light source lamp is turned on by a DC output of a power-factor improving circuit (PFC), and also to a projector using such a power supply device.
2. Description of the Related Art
In a conventional projector, an image is displayed on an image display device such as a liquid crystal display device, and an image display face of the image display device is illuminated with light of the light source lamp which is turned on, to project the image displayed on the image display device onto a screen. Such a projector is configured so that reflected light or transmitted light from the image display device is projected onto a screen through a projection lens. A light source lamp that is used in a usual projector is a vacuum discharge-based light source lamp such as a mercury lamp.
Such a conventional projector uses a power supply circuit in which a commercial power source of, for example, AC 100 V is used as an input power supply, the input power is full-wave rectified by a rectifying circuit, an output of the rectifying circuit is upconverted by a power-factor improving circuit (PFC), and the voltage upconverted by the power-factor improving circuit is converted to a predetermined voltage by a DC—DC converting circuit. A power supply circuit in which a power-factor improving circuit is disposed between a rectifying circuit and a DC—DC converting circuit is disclosed in JP-A-8-172773, JP-A-2000-224847, and JP-A-2001-333573.
The projector is configured so that a light source lamp is turned on by a lamp power supplying circuit to which an output of the power-factor improving circuit is supplied as an input power supply, and is configured so that components on the secondary side such as a CPU controlling the operation of a main unit are operated by an output voltage of the DC—DC converting circuit.
In such a conventional projector, even when the power-factor improving circuit is not operated (the power-factor improving circuit is in the stop state), the lamp power supplying circuit continues to supply the power to the light source lamp. In the state where the power-factor improving circuit is not operated, the output of the rectifying circuit is not upconverted. For example, the case where a power supply of AC 100 V is input will be considered. When the power-factor improving circuit is not operated, the input power supply of the lamp power supplying circuit is DC 140 V which is the output of the rectifying circuit, and, when the power-factor improving circuit is operated, the input power supply is DC 380 V.
When the power-factor improving circuit is not operated, therefore, the amount of light emitted from the light source lamp is reduced because of the lowered input voltage of the lamp power supplying circuit. As a result, the image displayed on the image display device is not properly projected onto the screen. The use of a light source lamp under this situation lowers the characteristics of the light source lamp, and causes a failure due to the increased load of the lamp power supplying circuit.